An important function that must be provided in optical networks is wavelength routing. In particular, wavelength routing is needed at each node, in order to combine and separate different wavelength channels. Typically, the router is realized in integrated form by using a waveguide grating, but a limitation of this router is its narrow bandwidth, which is typically much smaller than the channel spacing. This limitation is caused by the strong wavelength dependence of the output images produced by the grating, and it can be eliminated by replacing the grating with a combination of two gratings of opposite dispersions, as shown in U.S. Pat. No. 7,010,197 B2 which issued on Mar. 7, 2006, and U.S. Pat. No. 7,283,700, which issued on Oct. 16, 2007. By this technique the router produces, at the location of each output waveguide, a stationary image resulting in nearly maximum transmission having bandwidth nearly equal to the channel spacing. However, an undesirable feature of this technique is the large size of the router. In particular, the first patent requires between the two gratings many waveguide lenses, one for each output waveguide, and therefore this design is only feasible if the number N of output waveguides is small. On the other hand, only one lens is required by the second patent, but an undesirable feature in this case is the large spacing of the output waveguides. Therefore, also in this case, the router is only feasible if N is small.
Accordingly, the present patent proposes a new design that substantially increases the number of waveguides, without substantially increasing the size of the router. In the new design, a single waveguide lens is included between the two gratings, as in the previous patent. Now, however, each output waveguide is replaced by a set of several waveguides receiving different diffraction orders of the output stage.